A conventional method of forming containers of expanded resin, such as for example expanded polystyrene containers, consists in filling the cavity of a metal mold with expandable thermoplastic resin particles, for instance expandable polystyrene beads, heating the particles to be expanded and fused, and cooling the same. So-called expanded polystyrene containers produced in the above fashion are widely used in the distribution stage and consumption stage due to their combined properties of high forming accuracy, excellent heat insulation, and great strength-to-weight ratio. Containers for instant noodles, cups for beverage, etc. can be cited as familiar examples.
When expanded polystyrene containers are designed for containing a liquid substance, they must be produced such that the liquid substance contained therein does not leak. In addition, when the expanded polystyrene containers are formed in a slender and deep tapering shape, such as for example, cups for beverage etc., a so-called stacking height or the height of cups stacked as fitted in one another, should be as low as possible to improve the housing efficiency in storage and transportation of the cups. To reduce the stacking height, thin-walled containers should be made. However, if, in order to produce thin-walled containers, the quantity of resin particles filled in the cavity formed when the female member and the male member of a metal mold are closed, is reduced, the walls of the cups inevitably contain pin-holes or gaps whereby the water-tightness of the containers cannot be assured.
A method of forming thin-walled containers made of expanded resin having a satisfactory strength-to-weight ratio, while maintaining a mass equivalent to conventional thick-walled containers of expanded resin, is disclosed in Japanese Publication for Examined Patent Application No. 8744-1973 (Tokukosho No. 48-8744) and Japanese Publication for Unexamined Patent Application No. 142668-1975 (Tokukaisho No. 50-142668). Such a method consists in compressing and reducing the wall thickness of the containers prior to or after a cooling process, after the resin particles are expanded through heat. However, cups having a wall thickness equal to 0.5 mm such as for example thin-walled cups that can be employed in automatic vending machines of beverages, could not be formed with such a method. Moreover, the problem of water-tightness is left unresolved.
A method solving the problem of water-tightness and shortening the forming cycle time is disclosed in Japanese Publication for Unexamined Patent Application No. 153119-1988 (Tokukaisho No. 63-153119; Japanese Publication for Unexamined Patent Application No. 153119-1988 corresponds to U.S. Pat. No. 4,758,394). This method consists in conducting the forming process of preformed articles (hereinafter referred to as preforming process) and the forming process of finished articles (hereinafter referred to as finish-forming process), in a closed chamber designed such that the interior pressure thereof can be controlled. With this method, resin particles are expanded through heat during the preforming process in an atmosphere controlled to an atmospheric pressure such that the resin particles do not expand unrestrictedly. The preformed article is then locked into a metal mold having a spatial width smaller than the metal mold employed during the preforming process, and re-heated and mechanically compressed at the same time during the finish-forming process, thereby producing a thin-walled expanded resin cup. With this method the preformed article can uniformly be heated to be fused. This method thus enables the production of water-tight cups having a wall thickness equal to 0.5 mm. In addition, the preformed article is kept in a heated state while it is transferred to be subjected to the finish-forming process. Therefore, the time needed for re-heating during the finish-forming process is reduced and the forming cycle is shorter than a conventional forming cycle.
FIG. 17 schematically illustrates the forming process adopted in the forming method disclosed in Japanese Publication for Unexamined Patent No. 153119-1988 (Tokukaisho No. 63-153119, U.S. Pat. No. 4,758,394). Processes designated with the same step number in the preforming process and the finish-forming process, such as for instance S21 and S21' are conducted simultaneously.
During the preforming process, the female member and the male member of a preforming mold lock thereby forming a preformed article (S21'). The female member of the preforming mold is then lifted up to open the preforming mold (S23'). A cylinder connected to the male member of the preforming mold is then operated to move horizontally the male member whereon the preformed article is disposed toward the finish-processing side (S24'). The preformed article is released (S25') from the male member of the preforming mold that is then returned to its original position in the preforming processing side (S27'). The preforming process is thus completed.
Meanwhile, during the finish-forming process, the preformed article is transferred from the male member of the preforming mold to the female member of a finish-forming mold (S25). The female member of the finish-forming mold is temporarily lifted up (S26) and after the male member of the preforming mold is returned to its original position in the preforming processing side, the female member and the male member of the finish-forming mold lock, thereby enabling the preformed article to be compressed and formed into a finished article (S28). The male member of the finish-forming mold is then moved down (S29) and the finished article is discharged (S21). The female member of the finish-forming mold is lifted up (S23) after the male member of the finish-forming mold was moved up again to prevent the inner pressure of the closed chamber from decreasing (S22). The female member of the finish-forming mold then stays in a standby state until the arrival of another preformed article (S25). The finish-forming process cycle is thus completed.
With a method of forming containers of expanded resin through the fusion of resin particles such as described above, the finished articles cannot be prevented from having pinholes due to insufficient heating, insufficient expanding, etc. In addition, holes might also appear in the preformed article during the preforming process due to a defective action of a raw material feeder, or a defective fusion of the resin particles. There also might be some instances where crackles develop during the finish-forming process due to defective heating and/or cooling conditions. A test for detecting defective articles is thus indispensable. A conventional method for detecting defective articles consists in, for example, pouring a liquid substance such as water or other substance into the finished article and visually checking whether it leaks.
However, the above conventional method suffers from the drawback that the supply of the preformed article and the discharge of the finished article cannot be conducted efficiently within a short time during the finish-forming process. A shorter forming cycle is thus desirable.